1. Field of the Invention
The present invention concerns a method to create a difference image of two MR images by means of a magnetic resonance system. Moreover, the present invention concerns a correspondingly designed magnetic resonance system as well as a corresponding non-transitory electronically readable data storage medium.
2. Description of the Prior Art
New fields of application in magnetic resonance tomography are offered by the acquisition of MR data with very short echo times (<500 μs). It is thereby possible to show substances or tissue that cannot be shown by means of conventional sequences—for instance a (T)SE sequence (“(Turbo) Spin Echo”) or a GRE sequence (“Gradient Echo”)—since their T2 time is markedly shorter than the echo time and thus a corresponding signal from these substances or tissues has already decayed at the point in time of acquisition. For example, with echo times in the range of the corresponding decay time, it is possible to show bones, teeth or ice in an MR image although the T2 time of these subjects is in a range from 30-80 μs.
Sequences are known that enable a very short echo time. In addition to the radial UTE sequence (“Ultrashort Echo Time”) there is the approach to scan k-space at points at which the free induction decay (FID) is detected. Such a method is also designated as single point imaging since essentially only one raw data point or k-space point in k-space is detected per RF excitation.
One example of such a method for single point imaging is the RASP method (“Rapid Signal Point (RASP) Imaging”, O. Heid, M. Deimling, SMR, 3rd Annual Meeting, Page 684, 1995). According to the RASP method, a raw data point in k-space, whose phase was coded by gradients, is read out at a fixed point in time after the RF excitation relative to the “echo time” TE. The gradients are modified by means of the magnetic resonance system for each raw data point or measurement point, and k-space is thus scanned point-by-point.
According to the prior art, to create an image that shows only substances or tissue that have a very short T2 time (for example bone), it is typical to implement the RASP method twice, with the RASP method operating with such a short echo time TE in the first pass that bones, for example, still deliver a signal and with the RASP method operating with a correspondingly longer echo time TE in the second pass so that the bones no longer deliver a signal. Each pass of the RASP method respectively provides an image, so two images that are created in such a manner are subtracted from one another so that only tissues or substances having a very short T2 time are still shown in the resulting difference image.